The present invention relates to an improved method of manufacturing a gun barrel. More particularly, the present invention relates to an autofrettage method designed for the manufacturing of liquid cooled barrels subjected to high rates of fire.
For years large caliber guns have used residual stress to provide increased strength and fatigue life to the barrel. The method most often used to produce the favorable residual stress is called autofrettage which is a manufacturing process resulting in plastic deformation to the interior of the barrel. The autofrettage increases the elastic strength of the barrel, makes gross change in their resistance to fatigue and inhibits the rate of crack propagation.
The autofrettage plastic deformations can be created in a number of ways including explosive, hydraulic or mechanical means. For example, mechanical autofrettage utilizes a press to force an oversized mandrel through the bore of a pre-machined forging. This causes the material at the bore to yield in tension while allowing the material at the outside diameter to remain elastic. After the mandrel has passed through the bore, the relaxation of the material results in a distribution of residual stress that is compressive on the interior of the barrel. The magnitude of this residual stress is highly dependent on the amount of material yielding that is induced during this process, which is in turn governed by geometric tolerances and material properties.
Recently, the operational firing requirements for large caliber guns have dramatically increased. The emphasis on firing rates in excess of ten rounds per minute for extended periods has complicated barrel construction. High fire rates create a number of problems including xe2x80x9ccook-offxe2x80x9d of the ammunition propellant, projectile exudation, and increased tube wear. Currently, the gun barrel temperature is monitored during firing, whether in combat or training. When the barrel is judged to be too hot, the firing must be halted to allow the barrel to cool. Therefore, barrels must be cooled by air or liquid to remain operational.
There is a need then to develop barrels with extended life capable of handling the high rates of fire. In general, a balance must be achieved between the thermal stress produced by the cooling system and the residual stress produced by autofrettage. The ability to simply include cooling features in such newly designed larger caliber barrels is not straightforward due to the tremendous pressures created within the barrel, on the order of 60,000 psi. Typically, air cooled large caliber guns have been autofrettaged to a level where plastic deformation occurs throughout approximately 50% of the wall thickness. This practice is acceptable in traditional air cooled barrels where the thermal stresses in the barrel are significantly less than water-cooled barrels. However, air-cooling will not support the higher firing rates.
Therefore, a method is needed for balancing autofrettage stresses with the thermal stresses of a liquid cooled barrel. In the case of actively cooled guns, cooling of the barrels outside diameter induces significant thermal stresses that are incompatible with the stresses induced by traditional autofrettage methods. Previous design approaches therefore have either eschewed autofrettage, and designed to less demanding strength and fatigue requirements, or taken a midwall cooling approach. The creation of cooling channels within the barrel effectively reduces the thermal stresses and allows the level of over strain from the autofrettage process to approach that of a non-cooled design. However, the midwall design involves greater cost and manufacturing complexity.
There is a need then for a method of producing artillery barrels with the strength and fatigue life appropriate for current combat scenarios. The barrel must be able to withstand the pressure and stress associated with the high fire rates. Moreover, it would be desirable, based on cost and manufacturing complexity, to construct such a barrel using existing heat transfer methods so as to avoid the midwall cooling designs. The method should thus incorporate the expected barrel temperature profile and heat flux inputs from a worst case scenario when determining the level of autofrettage. Due to manufacturing tolerances, the method should be tailored for each barrel to further optimize the residual stress distribution to avoid bore collapse yet be as large as possible to maximize the fatigue life.
The present invention is a method for applying a mechanical autofrettage process to externally liquid cooled artillery barrels. Due to the temperature distribution expected within the barrel due to the rate of fire, the autofrettage process must be limited so as to avoid bore collapse. The level of autofrettage is determined based on the yield strength of the material as compared to the acceptable stress level of the barrel. The method requires creation of a set of autofrettage mandrels, tailored by diameter and taper geometry, which are selectively rammed down the machined barrel so as to create an acceptable residual stress profile. Mandrel selection is based on the yield strength of the forging samples of each individual barrel combined with the individual mapping of the inner diameter of the barrel.